In general, a conventional active-matrix liquid crystal display device includes a display portion provided with a plurality (N) of scanning signal lines GL(1) to GL(N), a plurality (M) of video signal lines SL(1) to SL(M) crossing the scanning signal lines, and a plurality (M×N) of pixel formation portions P(1,1) to P(N,M) arranged in a matrix in accordance with intersections of the scanning signal lines and the video signal lines, each pixel formation portion including a liquid crystal capacitance (also referred to as a “pixel capacitance”) Clc formed by a pixel electrode and an electrode (also referred to as a “common electrode”) opposing the pixel electrode, as shown in FIG. 2 to be described later. Each pixel electrode is provided between two video signal lines SL(m) and SL(m+1), and connected to the video signal line SL(m) via a TFT 10.
Also, in the liquid crystal display device, a plurality (N) of auxiliary capacitance lines CsL(n) are formed in parallel to the scanning signal lines GL(n), as shown in FIG. 1 to be described later, and in each pixel formation portion P(n,m), an auxiliary capacitance Ccs is formed between the pixel electrode and the auxiliary capacitance line CsL(n).
In each pixel formation portion P(n,m) of the active-matrix liquid crystal display device as described above, when the TFT 10 connected to the pixel electrode is brought into ON state (conductive state), voltage from the video signal line SL(m) is applied via the TFT 10, and when the TFT 10 is brought into OFF state (non-conductive state), the applied voltage is retained in the liquid crystal capacitance Clc (and an auxiliary capacitance Ccs), so that pixel display is achieved in accordance with the retained voltage (n=1, 2, . . . , N; m=1, 2, . . . , M).
Here, in this conventional liquid crystal display device, after the TFT 10 is brought into OFF state, the voltage applied to the auxiliary capacitance line CsL(N) is changed, thereby changing the potential of the pixel electrode conforming to the common electrode (to which constant voltage Vcom is applied). Such a potential change will be described in detail with reference to FIGS. 6 and 7.
FIG. 6 provides waveform charts of various signals for providing a white display in a conventional liquid crystal display device, and FIG. 7 provides waveform charts of various signals for providing a black display in the conventional liquid crystal display device. A video signal S(m) shown in FIGS. 6 and 7, which is a voltage signal applied to the video signal line SL(m), has in reality a predetermined voltage value in accordance with a pixel luminance from 0V to 4V, but for ease of explanation, the video signal S(m) is shown in FIG. 6 to have a voltage value corresponding to the white display (maximum luminance), and in FIG. 7 to have a voltage value corresponding to the black display (minimum luminance). Note that by setting the lowest voltage of the video signal S(m) to 0V, it becomes possible to simplify the configuration of the power supply circuit, thereby minimizing power consumption as a whole.
Here, when the potential Vcom of the common electrode is fixed because alternating current drive is required for preventing a liquid crystal layer from experiencing time degradation, the common potential Vcom is often set at a midpoint voltage (in the variation width) of the video signal. By doing so, the potential of the pixel electrode conforming to the potential of the common electrode can alternate. In this case also, the common potential Vcom is set at 2V.
However, in the case of normally black liquid crystal display devices, the applied voltage is typically required to be at least about 4V in order to maximize the transmittance of the liquid crystal layer. Therefore, without any modification, the above configuration is not appropriate because the voltage applied to the liquid crystal varies in the range of ±2V. Note that in the case of normally white TN mode liquid crystal display devices also, as the voltage applied to the liquid crystal increases, black is displayed more deeply, resulting in enhanced contrast, and therefore the applied voltage in the range of ±2V is not appropriate.
Accordingly, in the conventional liquid crystal display device, the potential of the pixel electrode is changed by driving the auxiliary capacitance line CsL(n). Specifically, as is apparent from the relationship of connections shown in FIG. 2 to be described later, the potential of the pixel electrode Epix, when conforming to the common electrode Ecom, changes in accordance with the ratio of the auxiliary capacitance to the sum of the liquid crystal capacitance value Clc and the auxiliary capacitance value Ccs, as the potential of the auxiliary capacitance line CsL(n) changes. For example, in the case of the white display shown in FIG. 6, when the potential of a signal supplied to the auxiliary capacitance line (hereinafter, the signal being referred to as the “auxiliary capacitance line drive signal”) Cs(n) is changed by 4V, the potential of the pixel electrode Epix is changed by 2V where the liquid crystal capacitance value Clc: the auxiliary capacitance value Ccs=1:1, i.e., Ccs/(Ccs+Clc)=0.5.
Accordingly, as shown in FIG. 6, the pixel electrode potential of the pixel formation portion P(n,m) is maintained at 4V when the scanning signal G(n) transitions from active state (where the TFT 10 is turned ON) to non-active state, and thereafter it rises by 2V to 6V due to the aforementioned potential change of the auxiliary capacitance line drive signal Cs(n). Since the common potential Vcom is 2V, the voltage applied to the liquid crystal layer at this time is 4V. Note that any potential change due to parasitic capacitance on the scanning signal line is not taken into account here.
Such a potential change occurs when pixel display is achieved with the white display (maximum luminance), but in the case of pixel display with the black display (minimum luminance), the potential change occurs in a different amount although in a similar manner. While liquid crystals in general have dielectric anisotropy, it is known that in the case of liquid crystals used for normally black liquid crystal display devices, the dielectric constant for the white display is higher than that for the black display, so that the liquid crystal capacitance value Clc for the white display is higher. Concretely, in the case of the aforementioned liquid crystal with Ccs/(Ccs+Clc)=0.5 for the white display, for example, Ccs/(Ccs+Clc)=0.75 for the black display.
Therefore, the pixel electrode potential of the pixel formation portion P(n,m) providing the black display is maintained at 0V when the scanning signal G(n) is brought into non-active state, as shown in FIG. 7, and thereafter rises by 3V, to 3V due to the aforementioned potential change (4V) of the auxiliary capacitance line drive signal Cs(n). Since the common potential Vcom is 2V, the voltage applied to the liquid crystal layer at this time is 1V. This voltage value is less than a liquid crystal threshold voltage value (typically, approximately 1.5V) at which liquid crystal molecules are driven, and therefore the optical transmittance of the liquid crystal layer becomes 0, which is suitable for the black display. The foregoing operation is the same for the pixel electrode potential of the pixel formation portion P(n+1,m) in the next row with an inverted polarity and for the pixel formation portion P(n,m) in the next frame. Such a liquid crystal display device in which auxiliary capacitance lines are driven with a fixed common potential Vcom is disclosed in, for example, Japanese Laid-Open Patent Publication No. 2001-83943. Hereinafter, such a conventional art example is referred to as the “first conventional art example”.
Also, there is a conventional liquid crystal display device in which the common electrode Ecom is driven every scanning period, thereby changing the common potential Vcom so as to have a potential (e.g., 0V or 4V) opposite in polarity to the video signal S(m). With this conventional configuration, it is possible to apply voltage to the liquid crystal within the range of ±4V.
Note that in the above conventional configuration, it is not always necessary to drive the auxiliary capacitance line, and there is a conventional liquid crystal display device in which the auxiliary capacitance line is driven every scanning period in the same phase as the common electrode. With this conventional configuration, it is possible to drive the auxiliary capacitance line at a proper potential, thereby suppressing flicker, image memory phenomenon, etc. Such a conventional liquid crystal display device is disclosed in, for example, Japanese Laid-Open Patent Publication No. 2-913. Hereinafter, such a conventional art example is referred to as the “second conventional art example”.
[Patent Document 1] Japanese Laid-Open Patent Publication No. 2001-83943
[Patent Document 2] Japanese Laid-Open Patent Publication No. 2-913